During assembly of components on printed circuit boards, soldering fluxes are applied to the board material to facilitate soldering. After soldering, remaining flux residue must be removed. The flux chemistry determines what fluid is suitable for removing the flux. Unlike some fluxes, rosin-based fluxes are not removable with a water wash, and require more aggressive solvents that have environmental disadvantages. Such solvents may generate flammable and environmentally undesirable vapors that must be safely contained, and may also require costly disposal methods because they cannot be drained into conventional sewer systems.
Cleaning systems employing an emulsion of solvent and aqueous solution have been employed to reduce environmental and safety concerns. A typical emulsion includes a solvent that is immiscible with the aqueous solution. Therefore, when sprayed on parts being cleaned, the solvent droplets and aqueous droplets each independently contact and dissolve different contaminants, providing effective cleaning. Existing systems are believed to employ emulsions that consist of 10 or 20 percent solvent, with the remainder being water or another aqueous solution.
In existing systems, the emulsion fluid is pumped from a single chamber, which is entirely drained into a wash chamber. The fluid is sprayed within the wash chamber to clean parts contained therein. Thus, the proportion of solvent in the emulsion mixture does not appreciably vary during the wash cycle. Any wash fluid remaining on the parts has the same solvent concentration as it initially did.
In general, increased solvent concentration provides more effective cleaning, but carries with it increased operational costs and environmental, health and safety concerns.
The primary objects of the invention are to provide:
1. A cleaning apparatus and method for effectively cleaning electronic assemblies.
2. A cleaning apparatus and method that conserves solvent and minimizes the generation of waste water, reducing costs and environmental impact.
3. A cleaning apparatus and method that is safe to operate and which avoids fire, explosion, and health hazards.
According to the present invention, the primary objects are achieved by providing a cleaning apparatus having a supply tank containing an aqueous fluid, with a supply pump connected to draw fluid from an intermediate level in the supply tank. The supply pump directs the fluid to a wash chamber containing a spray pump for spraying the fluid through a spray assembly onto parts to be cleaned. A valved drain line provides selectable drainage from the wash chamber to the supply tank.
According to the method of the present invention, the primary objects are achieved by operating the supply pump to draw a first portion of fluid from the supply tank, the first portion being from a level at or above the supply pump intake. This first portion includes all of the solvent and a limited quantity of the aqueous solution. The first portion of the fluid is then sprayed within the chamber onto the assemblies. To reduce the solvent concentration, the drain line valve is opened to permit some of the fluid to return to the supply tank. As fluid returns to the supply tank, it partially mixes with the aqueous fluid remaining in the supply tank. The supply pump continuously draws fluid from the tank to replace the drained amount. Initially, this replacement fluid has a significantly lower solvent concentration than the fluid that resides in the wash chamber. Accordingly, the solvent concentration in the wash chamber diminishes until the wash fluid in the wash chamber and supply tank is thoroughly mixed, and approaches a common solvent concentration. Thereafter, the wash fluid is drained from the wash chamber, and the parts may be rinsed by water.
Because the final solvent concentration of the wash fluid is substantially lower than the initial concentration, the rinse process is more effective, requires less water, and generates less waste. Nonetheless, the initial solvent concentration during the wash cycle is sufficiently high to provide effective cleaning that might not otherwise be possible at the concluding solvent concentration.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description that proceeds with reference to the accompanying drawings.